This application relies for priority upon Korean Patent Application No. 2002-07296, filed on Feb. 8, 2002, the contents of which are herein incorporated by reference in their entirety.
The present invention relates to methods for making a paste of materials used to form semiconductor xe2x80x9cbumpsxe2x80x9d or surface irregularities as part of a semiconductor package process. More specifically, the present invention relates to methods for making a composition or paste of materials used to form bumps wherein trace elements, namely those present in amounts less than about 10% by weight, have a substantially uniform, homogeneous distribution, throughout the paste.
Semiconductor devices have been rapidly miniaturized and their performance has been highly improved along with developments in silicon processing techniques. Thus, even in the field of semiconductor package techniques, flip chip package techniques have been introduced so that high-performance miniaturized devices are effectively packaged, and the applications for such flip chip package techniques have gradually increased.
However, in case of known flip chip package techniques, extensive studies have been made of methods of forming bumps for purposes of increasing throughput and lowering costs while maintaining high quality standards.
Conventional techniques for forming xe2x80x9cbumpsxe2x80x9d on semiconductor chips include e-beam evaporation, electroplating, electroless-plating, screen print, ball placement, and the like. These several methods have their own various advantages and disadvantages, as summarized in the table of FIG. 1.
Referring to the table of FIG. 1 and the table of FIG. 2, in the cases where a paste is composed of materials, one or more of which is present in trace amounts of less than about 10 wt. %, the most widely used method of forming bumps is that the desired materials are deposited using electroplating without loss. Although the electroplating method has the advantage of forming bumps with a fine pitch, it also has the disadvantage of causing low throughput. In addition, considerable know-how is required to successfully practice this method because there are many variable control parameters involved in composition control. Furthermore, when a bump is to be composed of single or binary components, such bump may be formed by a one-time deposition process. But, if a ternary bump composition is required, or a bump composition with more than three components, the throughput becomes very low because the deposition process must be repeated two or more times.
Based on low costs and high throughput, the screen print process or the ball placement process would ordinarily be the best method of forming multi-component bumps. However, the ball placement technique is possible only when a ball size is 300 xcexcm or more because a flip chip process has a restriction on pitches of balls.
In a successful screen print process, on the other hand, bumps can be formed only when the composition being used consists of 63Sn/37Pb. As lead-free products have recently attracted considerable attention as part of the trend toward environmentally-friendly manufacturing, it has become undesirable to use a lead-based paste to make bumps using the screen print process. This is also the case because bump compositions typically contain trace elements of less than 10 wt. % of the overall composition as illustrated in the table of FIG. 2 wherein the numbers refer to wt. % of each component of the known compositions.
As mentioned above, one of the easiest and least expensive ways of forming bumps is to use the screen print process. However, if the bump compositions contain components in trace amounts, it is virtually impossible to conformally mix powders to obtain a substantially uniform, homogeneous paste. Thus, according to a conventional method of making a paste for forming bumps, a paste is prepared by mixing powders of the components desired for constituting the bumps including the step of kneading the mixed powders with resin or flux to facilitate mixing. In this case, however, the distribution of trace elements in the mixed powders may become non-conformal. This results in low reliability and consistency of the resulting semiconductor products.
It is a feature of the present invention to provide methods of making a paste of multiple components used to form bumps for a semiconductor package process, which methods make it possible to form an alloy powder in which distribution of trace elements is substantially conformal.
It is another feature of the present invention to provide methods of making a paste of multiple components used to form bumps for a semiconductor package process, which methods make it possible to form substantially conformal bumps of a desired composition using a screen print process.
The objects of the present invention can be achieved by methods of making a paste of multiple components used to form bumps for a semiconductor package process comprising at least the steps of heating and fusing the component materials to form a fused alloy of the materials. The methods may further comprise the step of rapidly cooling the fused alloy to improve conformity of the resulting composition. In a further step, the cooled alloy composition may then be processed into a fine powder. Such alloy powder may be processed to be a paste-shape.
In the present invention, each of the materials, elements, or components of the alloy is preferably heated with the use of a separate furnace. The methods of this invention further comprise in another preferred embodiment alloying the materials, each of which has been fused with the use of a separate furnace, with the use of a mixing furnace to blend the components in the fused state.
In another embodiment of this invention, the materials are fused with the use of a single heating furnace to cause the components to be alloyed. In a further embodiment of this invention, the process for making the cooled alloy composition into a powder comprises an arc discharging step and/or a crushing step.
In another embodiment, the step of rapidly cooling the alloy composition comprises a first cooling step and a second cooling step. In the first cooling step, the fused alloy composition is slowly cooled to a first cooled temperature that is at or slightly higher than the melting point of the alloy composition but lower than melting points of each of the several separate component materials. In the second cooling step, the partially-cooled alloy composition is rapidly cooled from the first cooled temperature to a second cooled temperature which is at or below the melting point of the alloy composition. The step of rapidly cooling the alloy composition from the first cooled temperature to the second cooled temperature is preferably carried out in a vacuum state and/or in an inert gas ambient condition in order to prevent oxidization of the alloy composition. The earlier step of fusing the component materials may also preferably be carried out in a vacuum state and/or in an inert gas ambient condition in order to prevent oxidization of the component materials or possible contamination caused by impurities in the surrounding environment.
It will be understood that this invention may be embodied in different forms and should not be construed as limited to the specific embodiments set forth herein. Rather, the several embodiments herein described are merely illustrative of the invention, and the full scope of the invention will be apparent to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity.